1. Field of the Invention
The present invention relates to a spread illuminating apparatus of side light type, and particularly to a spread illuminating apparatus which includes a transparent resin plate composed integrally of a light conductor plate and a housing frame, and in which a means of fixing a light reflecting plate to the transparent resin plate has a flexible structure.
2. Description of the Related Art
A liquid crystal display (LCD) device is characterized by a small thickness, a small occupation volume, a light weight and the like and is used extensively in various electric products, such as a mobile telephone and a personal computer. Since a liquid crystal in the LCD device does not emit light by itself, an illuminating means is required when the LCD device is used at a place where a solar light or an illumination light is not fully available.
While the LCD device is requested to be further downsized, its display area is requested to be enlarged. In order to satisfy such requests at one time, the ineffective portion of a lighting means must be reduced, and also its power consumption is desired to be lowered, which is achieved by efficiently utilizing light from a light source.
Under the circumstance, recently, a spread illuminating apparatus has been developed in which a point light source, for example, a light emitting diode (LED) is used as a light source of a lighting means, or in which constituent members such as a light source, a light conductor plate and the like are fixedly disposed in place thereby efficiently utilizing light. Such the apparatus is generally requested to have a lower profile as a whole.
FIG. 11 shows a conventional spread illuminating apparatus of side light type in which LEDs 2 and a light conductor plate 1 having a substantially rectangular shape and adapted to receive light from the LEDs 2 are disposed in place by a housing frame 7 made of a white resin. Protrusions 12a and 12c are provided at each of side surfaces 11 and 12 of the light conductor plate 1 orthogonal to a side surface 8 at which the LEDs 2 are disposed while recesses 20a and 20c are provided at each of inner side surfaces of the housing frame 7 adapted to oppose the side surfaces 11 and 12 of the light conductor plate 1, and when the light conductor plate 1 is set in the housing frame 7, the protrusions 12a and 12c of the light conductor plate 1 engage respectively with the recesses 20a and 20c of the housing frame 7, whereby the light conductor plate 1 is fixedly disposed in place with respect to the housing frame 7 (refer, for example, to Japanese Patent Application Laid-Open No. 2005-302485).
In the engagement structure described above, however, a predetermined gap is required between the side surface 11/12 of the light conductor plate 1 and the inner side surface of the housing frame 7, and light loss attributable to the gap must be made up for by some measures. The white resin of the housing frame 7 has a smaller reflectance than a light reflecting plate which is constituted by a reflection film such as an enhanced specular reflector (ESR) (refer, for example, to Japanese Patent Application Laid-Open No. H9-506837), and some action must be taken to make up for light loss attributable to this low reflectance.
When the apparatus described above is further reduced in thickness, its entire rigidity becomes insufficient, and also it is likely to happen that the protrusions 12a and 12c of the light conductor plate 1 disengage from the recesses 20a and 20c of the housing frame 7.
FIG. 12 shows another conventional spread illuminating apparatus adapted to achieve a sufficient mechanical strength in the downsizing effort, which has an integrated structure of a light conductor plate portion 421 and a housing frame portion 422, where a pattern 426 is formed at a predetermined distance from light sources 410 (refer, for example, to Japanese Patent Application Laid-Open No. 2006-285171).
In the integrated structure described above, however, the housing frame portion 422 is to be made of a resin material of which the light conductor plate portion 421 is made and which has a high optical transmittance, and therefore light is caused to leak from the housing frame portion 422. In consideration of the light leakage problem, grooves 425 are provided at portions of the light conductor plate portion 421 located close to the housing frame portion 422, and light emitted from the light source 410 and traveling in the light conductor plate portion 421 is totally reflected by the groove 425.
Since polycarbonate or polymethylmethacrylate that is a common resin material for a light conductor plate has a refractive index of about 1.5 and has a relatively large critical angle, it may potentially happen that lot of lights emitted from the light source 410 and traveling in the light conductor plate impinge on the groove 425 at an angle smaller than the critical angle of the resin material depending on the design of constituent members, and the groove 525 does not sufficiently prevent the lights from leaking thus causing light loss.
Also, for preventing light leakage from the housing frame portion 422, a reflective film must be fixedly attached to the outer side surfaces of the housing frame portion 422 by some kind of method, and, for example, if a double face tape is used for fixed attachment, light is absorbed by the adhesive layers of the double face tape. Even if the reflective film can be fixedly attached without using a double face tape, light is inevitably absorbed and lost to some extent while traveling in the housing frame portion 422.
Referring to FIGS. 10A and 10B, in a spread illuminating apparatus in which a light conductor plate 101 is disposed inside a housing frame 102, and a light reflecting sheet 103 has its edge portion fixed to the housing frame 102 by a double face tape 104, the light reflecting sheet 103 undergoes warpage or undulation due to heat generated in the apparatus or coming from outside the apparatus, which results in lowering the brightness, causing light leakage, and deteriorating the aesthetic appearance. This warpage and undulation problem is attributable to the fact that the material of the light reflecting sheet 103 has a thermal expansion coefficient different from that of the material of the housing frame 102 (the material of the light reflecting sheet 103 has a larger thermal expansion coefficient than other constituent members).